1. Field of the Invention
The present invention concerns an optical article such as a lens or a mirror, an exposure apparatus or optical system such as a stepper having it, and a process for producing the optical article. More particularly, the invention relates to an optical thin film for an optical article suitably applicable to an exposure apparatus or optical system using an excimer laser, and a process for producing it.
2. Related Background Art
Optical articles, including lenses, mirrors, and optical filters, are used, for example, in optical apparatus such as cameras, telescopes, and microscopes. These optical articles have an antireflection film or a reflection-enhanced film on the surface thereof for prevention of reflection or for enhancement of reflection.
An exposure apparatus is a kind of an optical apparatus equipped with such an optical article. The exposure apparatus is used in the fabrication steps of semiconductor integrated circuits or photomasks for fabricating them. A typical example of an exposure apparatus in this field is an exposure apparatus called a stepper.
An illumination light source in such an exposure apparatus, conventionally used, was a super-high pressure mercury lamp, a xenon-mercury arc lamp, or the like for emitting the g-line (435.8 nm), the h-line (404.7 nm), and the i-line (365 nm). However, an attempt has recently been made to use lasers emitting far-ultraviolet rays (200 to 260 nm) or emitting a beam of high power and narrow spectral width in order to realize high exposure processing capability per unit time (throughput) and uniform illumination characteristics on an exposed body such as a wafer. Among others, an excimer laser is one of desirable light sources, because it emits light in a very narrow spectral width and at high output power.
One of known optical articles for an excimer laser is the one having an antireflection film deposited by vacuum vapor deposition as disclosed in Japanese Laid-open Patent Application No. 63-113501 or Japanese Laid-open Patent Application No. 63-113502.
However, for example, if a lens having sufficient optical characteristics in the optical system for visible light was used in an optical system for an excimer laser, it was sometimes difficult to maintain optical characteristics sufficient to be applied to practical use. Specifically speaking, transmission characteristics, particularly, of the optical thin film provided over the surface of a lens or mirror were not sufficient to make the best use of the advantages of the excimer laser, and there was a problem in the durability thereof.
In addition, since the optical articles for an exposure apparatus were required to have a surface formed at very high accuracy, strict control of temperature conditions was necessary in forming the optical thin film. Thus, film forming techniques generally regarded as being preferable were not able to be used as they were in fabricating such optical articles.
As explained above, in order to fabricate an optical article that is durable against the use of an excimer laser, the optical article must be designed based on a novel idea and a novel approach.